When you purchase shares using reinvested distributions, it's as if the distributions were paid to you in cash, and then you immediately used that cash to purchase new shares.
The reinvested dividend distribution is included in the amounts shown on the 1099-DIV that you receive from your mutual fund. You don't need to make any special entries related to the reinvested dividend distribution when you enter the 1099-DIV in the program. But you do need to keep track of the reinvested dividend distributions affect on your basis.